# Quantum mechanics - spin operator eigenvalue probability?

1. May 23, 2010

### jeebs

Hi,
I have this problem on a past exam paper I am having some trouble with:

"in the conventional basis of the eigenstates of the Sz operator, the spin state of a spin-1/2 particle is described by the vector:

$$u = \left( \stackrel{cos a}{e^i^b sina} \right)$$ where a and B are constants.

find the probability that a measurement of the y-component of the spin of the particle will yield the result $$0.5\hbar$$ ."

For the life of me I cannot work out how to write out matrices legibly on this thing, so I will summarize what is bothering me. I am given pauli matrices $$\sigma_x_,_y_,_z$$that I cannot write out properly, and the spin operator is given by $$S_i = i\hbar\sigma_i$$.

In the question I am given the vector u, which is apparently expressed in the basis of Sz eigenstates.

Am I justified in putting this vector u into an eigenvalue equation $$S_{y} u = a_{y} u$$ ,

where ay is my eigenvalue, when the vector I would be operating on is made from a basis of eigenstates of another operator (Sz)?

I tried this and got two equations for ay, neither of which gives $$a_y = 0.5\hbar$$.

does this mean I can conclude that there is zero probability of finding the y-component of the spin being equal to $$0.5\hbar$$ ?

or do I somehow have to wangle it so that I get another vector (not u) that is in the Sy eigenstate basis?

thanks.

PS. sorry, this crazy thing will not let me change something 5 lines up where I should have said
"Am I justified in putting this vector u into an eigenvalue equation S_y u = 0.5\hbar u"

Last edited: May 23, 2010
2. May 23, 2010

### gabbagabbahey

You tell me. If $u$ is an eigenvector of $S_z$, is it also an eigenvector of S_y[/itex]?

3. May 23, 2010

### jeebs

are you actually asking me, do these two operators commute? and if their commutator [Sy, Sz] = 0, then yes they do?

is that right, if the eigenvector of one operator is equal to the eigenvector of another, then the operators commute?

4. May 23, 2010

### gabbagabbahey

Yes. So, do $S_z$ and $S_y$ commute?

5. May 23, 2010

### jeebs

not by my calculations, although I just woke up 5 minutes ago. I get [Sy,Sz] is equal to
(hbar^2)/4 multiplied by a 2x2 matrix containing zeros on the leading diagonal and 2i elsewhere.

6. May 23, 2010

### gabbagabbahey

Right, they don't commute; $[S_y,S_z]=i\hbar S_x$. So, $u$ is clearly not an eigenstate of $S_y$.

What are the eigenstates of $S_y$?

7. May 23, 2010

### jeebs

i got that one of them was the column vector, say, x:
x_1

ix_1

and the other vector y:
y_1

-iy_1

but I was not convinced by this because, if the upper component is say, x1, and I choose x1 = 1, then if I go to normalize it i do
1/sqrt[(1^2) + (i^2)] = 1/(1-1) = 0, so how can this be right if Im getting a normalization constant of zero?

8. May 23, 2010

### vela

Staff Emeritus
You forgot to take the conjugate. You should be calculating $\langle x|x \rangle = x^\dagger x$, not $x^T x$.